Liquid natural gas is stored in large tanks at or close to ambient pressure and at cryogenic temperatures, the liquid in the tank being cooled as energy in the liquid is lost by some of the liquid boiling off as gas. Published PCT Application WO 2004/001280 describes such a tank. In order to reduce the loss of gas to a minimum, the walls, base and top of the storage tank are thermally insulated.
The base of a tank developed from the tank referred to above comprises a concrete footing, on which an outer metal plate is laid. A thermally insulating layer of foamed glass blocks is laid on the outer metal plate. A concrete base is then laid on the insulating layer, to form the bottom of an inner tank. A metal base is then built up from welded plates, to extend over the concrete base. In an embodiment of the invention, an outer edge of the metal base has a thickened region on which at least a part of the inner tank sidewall stands. The metal base of the inner tank is joined to a metal layer in the sidewall of the tank, to contain the liquid in the inner tank.
A cryogenic liquid tank conventionally comprises an inner tank having a sidewall including a metal layer joined to the metal base to contain the LNG, and an outer tank surrounding the inner tank and spaced from it.
The tank according to the invention comprising the bottom and the sidewall thus can be considered to be a cavity wall comprising an inner leaf forming the bottom and the sidewall of the inner tank, and an outer leaf forming the bottom and the sidewall of the outer tank. The space between the inner and outer leaves of the sidewall is filled with an insulating material such as perlite.
In order best to cope with the hydrostatic pressures exerted by the liquid on the inner leaf of the tank sidewall, and to facilitate fabrication of the tanks, LNG storage tanks conventionally have a circular shape in plan, and vertical walls forming a cylindrical shape. The top edge of the outer leaf of the sidewall of the tank is reinforced by a ring beam structure to take up the forces exerted by a steel and concrete dome structure placed over the top of the tank to rest on the outer leaf of the sidewall. The steel structure of the dome may be prefabricated as a single piece and lifted into place intact. Alternatively, and particularly when slip-forming is used to construct the tank sidewall, the steel structure of the dome may be prefabricated as a series of sectors, and assembled once the outer leaf of the sidewall of the tank has been raised to the desired height. The steel structure of the dome is not a significant weight and may simply rest on the outer leaf of the tank sidewall. The ring beam structure installed around the top of the wall is reinforced circumferentially to withstand the hoop stresses produced when the dome is finished with a concrete layer.
An insulating layer on top of the inner tank is usually arranged on a comparatively light lid structure which is suspended from the dome, to reduce heat influx into the surface of the liquid gas stored in the tank while being permeable to gas boiling off the liquid surface.
In the prior art, the inner leaf of a storage tank sidewall was fabricated from thick sheet metal in order to provide strength and liquid-tightness. Steel plates used for such purpose in such background art cryogenic tanks may be up to 38 mm thickness according to the inventors' knowledge. The thick sheet metal inner leaf was joined to the metal layer of the base by welding. However, the thick metal plates are expensive to produce and to shape and take time to join together, making this form of construction expensive.
In the prior art referred to above, the inner leaves of tank walls have been constructed by the use of a compact sandwich construction for the inner leaf, in which the inner leaf comprises a first erected slip-formed concrete inner layer surrounded by a thin metal layer, which in turn is surrounded by a subsequently slip-formed concrete outer layer. This sandwich construction enables the inner leaf to have a thinner metal layer than the “all-metal” inner leaf of the earlier tanks. Furthermore, the sandwich construction eliminates the disadvantage that when the liquid gas is placed in the tank, the cryogenic temperatures cause the liner to contract away from the outer concrete layer, stressing the attachments which fix the steel liner to the concrete.
At its lower edge, the thin metal layer is welded to a horizontal ring-shaped metal base plate forming the base of the inner leaf, the ring-shaped metal plate welded along its inner periphery to an outer periphery of the metal layer of the base to provide fluid-tightness.
The inner leaves of tank walls using this sandwich construction are usually made by a slip-casting process, in which a planar base structure is formed, a slip mould for an inner concrete structure is initially assembled on the base structure, and the inner concrete structure is slip formed to the desired height of the tank. Subsequently, the metal layer is arranged on the outer surface of the inner concrete structure, and then a slip form for the outer concrete structure of the inner leaf is arranged and slip-forming of this outer concrete structure of the inner leaf is carried out until the desired height of the inner leaf is reached. The metal layer is built up of lap-welded steel plates which may be erected as the inner concrete layer of the inner leaf is raised. The inner concrete layer, the metal layer, and the outer concrete layers of the inner leaf of the wall are thus formed sequentially according to the prior art.
In all of the previous techniques, however, the wall of the tank is a vertically-oriented cylinder of circular horizontal cross-section, with vertical side walls.
One limiting factor in the construction of such tanks is the size of the dome. A main limiting factor is the size of the dome and the generally horizontal forces induced on the top of the sidewall by the weight of the dome. Another limiting factor is the internal bending moment induced in a widely spanning dome.
Generally, there are two alternative ways of forming the dome on a cylindrical (vertical-wall) tank. The first is by forming the outer leaf of the tank sidewall, then constructing the steel structure of the dome within the bottom of the outer leaf, lifting the steel structure to its final position at the top of the outer leaf and attaching the steel structure there, and then covering the steel structure of the dome with a concrete layer to form the finished roof of the tank. Subsequently, the inner leaf of the tank sidewall is formed. Clearly, the inner leaf may only be formed after the roof has been lifted into place on top of the outer leaf.
Alternatively, the roof may be formed by slip-forming at least the outer leaf, and then hoisting the steel structure of the roof in sections into place on top of the outer leaf, and then forming the concrete layer to complete the roof. The steel sections of the dome may only be lifted into place when weather conditions are calm, and thus construction schedules are easily disrupted. These limiting factors in practice determine the diameter of the tank.
The present invention seeks to provide a structure for LNG storage tanks which allows an increased volume of LNG to be stored in a tank having the same height and base footprint or having the same roof span as a conventional cylindrical tank.
A second objective is to provide an LNG storage tank which allows having the same or even larger volume of LNG stored in the tank while having the same height and footprint as a conventional tank, but having a reduced roof span.
A further objective is to provide a base structure for a storage tank with improved mechanical strength and improved thermal insulation compared to prior art base structures. The base structure of the invention may be used in conjunction with sidewall structures whose inner leaves are of the sandwich type.